Seismotectonic and seismicity of Makran, a bimodal subduction zone, SE Iran

Abstract

The present study combined spatial distribution and mechanism of seismicity, seismicity parameters, geomorphology in satellite imagery and tectonic features in Makran inland to show that the Makran low angle thrust fault is divided into two separate segments. Depth dispersal of earthquakes, especially relocated events, shows that the sinking plate is steeper in the west than in the east of Makran. Significant facts were identified from the variation of Moho depth between west and east, slip rates of the inland faults and crustal shortenings in the north of Makran. The Sistan suture zone plays a significant role in dividing the overriding plate into a non-deforming Helmand block and a less deforming Lut block. These blocks have different resistances against the northward subduction. Large and great events, such as the 1989 Chabahar (MW 5.5) and 1945 Gwadur (MW 8.1), occurred in the south of subduction fronts of the western and eastern Makran, respectively. They occurred in shallow depth and were the pieces of evidence of weak and strong coupling between the oceanic and continental crusts on a low angle and a very low angle reverse faults, respectively. Spectral analysis and depth determination of the instrumental and pre instrumental earthquakes, respectively, helped to separate some plate boundary and intraplate events. This separation, geometry and non-characteristic behavior of the subduction zone, revealed from spatial and temporal distribution of the earthquakes, probably dedicate a mature Aleutian type for the subduction zone in the east. Missing great (M > 6.1) earthquakes, less slip rate of the continental faults and steeper dipping of the sinking plate in the west probably indicate a Kurile type for the Iranian part of subduction. Makran is a bimodal subduction, because geomorphological and seismic pieces of evidence indicate a NNW surficial and right lateral fracture zone in the continental crust and a NW deep and left lateral fracture zone in the oceanic crust, respectively. The fractures and geometry of subductions showed that the eastern Makran has seismotectonically a wider area for potential coupling of a larger earthquake than the western Makran.